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1.
Air and water velocity fields have been simulated during natural convection, using a two-dimensional volume of fluid (VOF)
model. The results have shown that during unstable thermal stratification, the root-mean-square (RMS) airside velocities are
an order of magnitude higher than the RMS waterside velocities, whereas, during the stable thermal stratification, the velocity
magnitudes are comparable for air and water sides. Furthermore, the magnitude of the air velocity changed more rapidly with
the change in the bulk air–water temperature difference than the water velocity, indicating that the air velocities are more
sensitive to the bulk air and water temperature difference than the water velocities. A physical model of the heat and mass
transfer across the air–water interface is defined. According to this model, the vortices on the air and water sides play
an important role in enhancing the heat and mass transfer. Due to the significance of the flow velocities in the transport
process, it has been proposed that the correlations for the heat and mass transfer during natural convection should be improved
by incorporating the flow velocity as a parameter. 相似文献
2.
Optical distortions have previously prevented non-intrusive measurements of dissolved oxygen concentration profiles by Laser
induced fluorescence (LIF) to within 200 μm of the air–water interface. It is shown that by careful experimental design, reliable
measurements can be obtained within 28 μm of moving air–water interfaces. Consideration of previously unidentified optical
distortions in LIF imagery due to non-linear effects is presented that is critical for robust LIF data processing and experimental
design. Phase resolved gas flux measurements have now been accomplished along wind forced microscale waves and indicate that
the highest mean gas fluxes are located in the wave troughs. The local mean oxygen fluxes as determined by LIF techniques
can be reconciled to within 40% of those obtained by bulk measurement in the water. These data provide a new perspective on
wind-wave enhancement of low solubility gas transfer across the air–water interface. 相似文献
3.
4.
A method for observing near-surface fluctuations in pH caused by a water–air flux of carbon dioxide under conditions of ambient
atmospheric carbon dioxide levels is developed and tested. Peaks in fluorescence intensity measured as a function of pH and
turbulence are shown to be consistent with predictions from a chemical kinetics model of CO2 exchange. The square root of the frequency of the pH fluctuations scale linearly with independently measured bulk air–water
gas transfer velocities in agreement with surface divergence models for air–water gas transfer. These data indicate that the
method proposed here is tracking changes in near-surface CO2 concentrations. This laser-induced fluorescence method can be used to study the air–water exchange of CO2 in wind-wave tunnels without the need for elevated CO2 concentrations in the gas phase. 相似文献
5.
A PIV based technique is developed to perform flow measurements in the vicinity of the air–water interface of a submerged
confined jet. Both the interface movement and the velocity field immediately beneath it are measured simultaneously. A detailed
turbulence structure in the surface influence region is thus obtained. Flow parameters evaluated without and w.r.t. the interface
are quantified and compared against previous works obtained using the conventional Eulerian-based instrumentation which do
not account for the interface fluctuation, and checked against analytical model characterising the turbulence close to a assumed
flat air–water interface.
Received: 15 March 1998/Accepted: 19 October 1998 相似文献
6.
João Eduardo Borges Nuno H. C. Pereira Jorge Matos Kathleen H. Frizell 《Experiments in fluids》2010,48(1):17-31
The development of a three-hole pressure probe with back-flushing combined with a conductivity probe, used for measuring simultaneously
the magnitude and direction of the velocity vector in complex air–water flows, is described in this paper. The air–water flows
envisaged in the current work are typically those occurring around the rotors of impulse hydraulic turbines (like the Pelton
and Cross-Flow turbines), where the flow direction is not known prior to the data acquisition. The calibration of both the
conductivity and three-hole pressure components of the combined probe in a rig built for the purpose, where the probe was
placed in a position similar to that adopted for the flow measurements, will be reported. After concluding the calibration
procedure, the probe was utilized in the outside region of a Cross-Flow turbine rotor. The experimental results obtained in
the present study illustrate the satisfactory performance of the combined probe, and are encouraging toward its use for characterizing
the velocity field of other complex air–water flows. 相似文献
7.
Experimental assessment of scale effects affecting two-phase flow properties in hydraulic jumps 总被引:2,自引:0,他引:2
A hydraulic jump is the rapid transition from a supercritical to subcritical free-surface flow. It is characterised by strong
turbulence and air bubble entrainment. New air–water flow properties were measured in hydraulic jumps with partially developed
inflow conditions. The data set together with the earlier data of Chanson (Air bubble entrainment in hydraulic jumps. Similitude
and scale effects, 119 p, 2006) yielded similar experiments conducted with identical inflow Froude numbers Fr
1 = 5 and 8.5, but Reynolds numbers between 24,000 and 98,000. The comparative results showed some drastic scale effects in
the smaller hydraulic jumps in terms of void fraction, bubble count rate and bubble chord time distributions. The present
comparative analysis demonstrated quantitatively that dynamic similarity of two-phase flows in hydraulic jumps cannot be achieved
with a Froude similitude. In experimental facilities with Reynolds numbers up to 105, some viscous scale effects were observed in terms of the rate of entrained air and air–water interfacial area. 相似文献
8.
The forced convection heat transfer with water vapor condensation is studied both theoretically and experimentally when wet
flue gas passes downwards through a bank of horizontal tubes. Extraordinarily, discussions are concentrated on the effect
of water vapor condensation on forced convection heat transfer. In the experiments, the air–steam mixture is used to simulate
the flue gas of a natural gas fired boiler, and the vapor mass fraction ranges from 3.2 to 12.8%. By theoretical analysis,
a new dimensionless number defined as augmentation factor is derived to account for the effect of condensation of relatively
small amount of water vapor on convection heat transfer, and a consequent correlation is proposed based on the experimental
data to describe the combined convection–condensation heat transfer. Good agreement can be found between the values of the
Nusselt number obtained from the experiments and calculated by the correlation. The maximum deviation is within ±6%. The experimental
results also shows that the convection–condensation heat transfer coefficient increases with Reynolds number and bulk vapor
mass fraction, and is 1∼3.5 times that of the forced convection without condensation. 相似文献
9.
This paper is a contribution to the development of an original technique for measuring the in-cylinder equivalence air–fuel
ratio. The main objective was to construct an instrument able to furnish instantaneous values of hydrocarbon concentration
for many consecutive cycles at a definite location, especially at the spark plug location. The probe is based on a hot-wire-like
apparatus, but involves catalytic oxidation on the wire surface in order to be sensitive to the hydrocarbon concentration.
In this paper, we present the different steps needed to develop and validate the probe. The first step focuses on the geometric
configuration to simplify as much as possible the mass transfer phenomena on the wire. The second step is a parametric study
to evaluate the sensitivity, confidence and lifetime of the wire. By physical analysis, we propose a relationship between
the electrical signal and the air–fuel equivalence ratio of the sampled gases. The third step is the application of the probe
to in-cylinder motored engine measurements, which confirms the ability of the technique to characterise, quantitatively, the
homogeneity of the air–fuel mixture, especially during the compression stroke. This work points out that the global sensitivity
is estimated at 4 V per unit of equivalence air–fuel ratio and the response time is estimated at about 400 μs. The equivalence
air–fuel ratio range is from pure air to 1.2. Experiments show that it is necessary to calibrate the system before use because
of the existence of multiple catalysis states. The probe presents advantages associated with its simplicity, its low cost
and its direct engine application without any modifications.
Received: 1 November 2000 / Accepted: 30 May 2001 相似文献
10.
In high-velocity open channel flows, the measurements of air–water flow properties are complicated by the strong interactions
between the flow turbulence and the entrained air. In the present study, an advanced signal processing of traditional single-
and dual-tip conductivity probe signals is developed to provide further details on the air–water turbulent level, time and
length scales. The technique is applied to turbulent open channel flows on a stepped chute conducted in a large-size facility
with flow Reynolds numbers ranging from 3.8E+5 to 7.1E+5. The air water flow properties presented some basic characteristics
that were qualitatively and quantitatively similar to previous skimming flow studies. Some self-similar relationships were
observed systematically at both macroscopic and microscopic levels. These included the distributions of void fraction, bubble
count rate, interfacial velocity and turbulence level at a macroscopic scale, and the auto- and cross-correlation functions
at the microscopic level. New correlation analyses yielded a characterisation of the large eddies advecting the bubbles. Basic
results included the integral turbulent length and time scales. The turbulent length scales characterised some measure of
the size of large vortical structures advecting air bubbles in the skimming flows, and the data were closely related to the
characteristic air–water depth Y
90. In the spray region, present results highlighted the existence of an upper spray region for C > 0.95–0.97 in which the distributions of droplet chord sizes and integral advection scales presented some marked differences
with the rest of the flow. 相似文献
11.
An experimental study has been performed to improve the understanding of the initial air–liquid interaction in the near field
of an air-blasted breaking water sheet. For the first time, planar laser-induced fluorescence (PLIF) has been used to visualize
the air-flow field, seeding the air streams with acetone vapor. Mie scattering from the liquid sheet, together with the acetone
fluorescence signal has enabled simultaneous determination of the instantaneous water sheet location and the air-flow structures.
The two-phase flow visualization has revealed detachment of the air boundary layer over the air–water interface behind the
zones of strong curvature. The pressure field induced by these vortices has been identified as a cause of the enhanced sheet
flapping and the instability growth.
Received: 30 October 2000/Accepted: 29 March 2001 相似文献
12.
Although a fundamental physical parameter, surface tension is difficult to measure. Common tensiometry inaccuracy comes from
failure to control air–liquid–solid contact conditions, or account for liquid meniscus geometry and buoyancy corrections.
This paper describes an in situ tensiometry technique, based on withdrawal of a thin-walled tube from the liquid interface,
that enforces a known air–liquid–solid contact condition. This technique can be pursued at any level of experimental hygiene.
Experimental results for filtered tap water, an alcohol–water solution, and a surfactant–water solution show that results
repeatable to three significant digits are obtained with modest effort for a variety of geometrical parameters.
Received: 7 October 1997/Accepted: 23 April 1998 相似文献
13.
A dual-probe hot-film anemometry technique has been developed to measure multiple gas-bubble velocities corresponding to
different gas-bubble size groups in air–water flows. A data reduction scheme using wavelet analysis combined with a phase-detection
technique is used to discriminate the hot-film anemometer output signals into signals corresponding to different bubble size
groups. The phase and bubble size discrimination is based on the magnitude and time derivative of the signal, and the streamwise
length of the gas bubbles. A cross-correlation between the discriminated signals from the two probes yields an average time
difference of arrival of the gas bubbles at the two sensor locations. The velocities are estimated from the distance between
the sensors and the time difference of arrival. The mean bubble size is estimated from the chord length distribution. Measurements
performed in vertical-up air–water slug flow show the technique to be a viable method for obtaining bubble velocity and size
information. The velocity measurements from the hot-film anemometry are corroborated using a high-speed quantitative flow
visualization system.
Received: 22 December 1999/Accepted: 8 May 2001 相似文献
14.
A detailed numerical study is carried out to investigate fluid flow and heat transfer characteristics in a channel with heated
V corrugated upper and lower plates. The parameters studied include the Reynolds number (Re = 2,000–5,500), angles of V corrugated plates (θ = 20°, 40°, 60°), and constant heat fluxs (q″ = 580, 830, 1,090 W/m2). Numerical results have been validated using the experimented data reported by Naphon, and a good agreement has been found.
The angles of V corrugated plates (θ) and the Reynolds number are demonstrated to significantly affect the fluid flow and the heat transfer rate. Increasing the
angles of V corrugated plates can make the heat transfer performance become better. The increasing Reynolds number leads to
a more complex fluid flow and heat transfer rate. The numerical calculations with a non-equilibrium wall function have a better
accuracy than with a standard wall function for solving high Reynolds numbers or complex flow problems. 相似文献
15.
In this work a simplified calculation method taking into account the effect of mass transport on the heat transfer coefficient
(HTC) during boiling of multicomponent mixture has been elaborated. The calculation results were compared with own experimental
data for ternary system methanol–isopropanol–water and Grigoriev data [1] (acetone–methanol–water). The experiments were performed in different hydrodynamic conditions such as: pool boiling and
liquid evaporation at the free surface of the falling film. The experimental data covered wide range of heat fluxes from 6
to 30 kW/m2 in the case of liquid evaporation from the falling film and from 30 to 240 kW/m2 for pool boiling. The analysis of the results indicates that the mass transfer resistance in the liquid phase caused a significant
reduction of experimental value HTC in comparison to so-called ideal HTC. 相似文献
16.
An imaging technique that uses backlighting has been developed to measure drop sizes in annular two-phase flows with small
concentrations of drops in the gas phase. Advantages over conventional photography are realized in that data collection and
analysis times are shortened considerably, and consistent unbiased results can be expected. A magnification of 1.9 was used
to measure drops above 50 μm. A drop size distribution was obtained for an air–water system as a superficial gas velocity
of 30 m/s and a liquid flow of 20 g/s. The data are used to substantiate a theory for the rate of deposition.
Received: 6 February 1997/Accepted: 3 February 1998 相似文献
17.
In this study, we carried out a numerical simulation of transient heat transfer in a composite passive system consisting of
air–phase change material–air, arranged as a rectangular enclosure. The vertical boundaries of the enclosure are isothermal
and the horizontal ones adiabatic. The enthalpy formulation with a fixed grid is used to study the process of phase change
with liquid–solid interface zone controlled by natural convection. The flow in this zone is simulated by a model based on
the Darcy porous medium. The numerical solution of the mathematical model is done using finite difference–control volume algorithm.
The influence of the geometrical and thermal parameters is studied. It is found that subcooling coefficient is the most important
parameter influencing heat transfer, and for a given subcooling, there is an optimum phase change partition thickness. 相似文献
18.
Steady convective mass transfer to or from fluid interfaces in pores of angular cross-section is theoretically investigated.
This situation is relevant to a variety of mass transport process in porous media, including the fate of residual non-aqueous
phase liquid ganglia and gas bubbles. The model incorporates the essential physics of capillarity and solute mass transfer
by convection and diffusion in corner fluid filaments. The geometry of the corner filaments, characterized by the fluid–fluid
contact angle, the corner half-angle and the interface meniscus curvature, is accounted for. Boundary conditions of zero surface
shear (‘perfect-slip’) and infinite surface shear (‘no-slip’) at the fluid–fluid interface are considered. The governing equations
for laminar flow within the corner filament and convective diffusion to or from the fluid–fluid interface are solved using
finite-element methods. Flow computations are verified by comparing the dimensionless resistance factor and hydraulic conductance
of corner filaments against recent numerical solutions by Patzek and Kristensen (J. Colloid Interface Sci 236, 305–317 2001). Novel results are obtained for the average effluent concentration as a function of flow geometry and pore-scale
Peclet number. These results are correlated to a characteristic corner length and local pore-scale Peclet number using empirical
equations appropriate for implementation in pore network models. Finally, a previously published “2D-slit” approximation to
the problem at hand is checked and found to be in considerable error. 相似文献
19.
Most environmentally important gases (such as oxygen or carbon dioxide) have low solubility. The transfer process of such
gases across the air–water interface is controlled by molecular diffusion and turbulence, concentrated within a very thin
layer in the water side. A challenge for a better understanding of the problem is to be able to elucidate the transport processes
happening within this thin layer. We used non-invasive measurement techniques to study the interaction between the gas transfer
process and the hydrodynamic condition in the water phase with two different turbulence forcing mechanisms, namely grid-stirred
and buoyant-convective turbulence. The experimental setup for the turbulence generation and the measurement techniques are
described. The different dominating turbulent transport mechanisms of the two cases were discussed, and a comparison of the
different measured scales between the two cases is also presented. 相似文献
20.
The transition from supercritical to subcritical open channel flow is characterised by a strong dissipative mechanism called a hydraulic jump. A hydraulic jump is turbulent and associated with the development of large-scale turbulence and air entrainment. In the present study, some new physical experiments were conducted to characterise the bubbly flow region of hydraulic jumps with relatively small Froude numbers (2.4 < Fr1 < 5.1) and relatively large Reynolds numbers (6.6 × 104 < Re < 1.3 × 105). The shape of the time-averaged free-surface profiles was well defined and the longitudinal profiles were in agreement with visual observations. The turbulent free-surface fluctuation profiles exhibited a peak of maximum intensity in the first half of the hydraulic jump roller, and the fluctuations exhibited some characteristic frequencies typically below 3 Hz. The air–water flow properties showed two characteristic regions: the shear layer region in the lower part of the flow and an upper free-surface region above. The air–water shear layer region was characterised by local maxima in terms of void fraction and bubble count rate. Other air–water flow characteristics were documented including the distributions of interfacial velocity and turbulence intensity. The probability distribution functions (PDF) of bubble chord time showed that the bubble chord times exhibited a broad spectrum, with a majority of bubble chord times between 0.5 and 2 ms. An analysis of the longitudinal air–water structure highlighted a significant proportion of bubbles travelling within a cluster structure. 相似文献